1. Field of the Invention
This invention relates to a fixing device and an image formation apparatus, such as a copier, a printer, a facsimile, or a multifunctional apparatus having several of these capabilities, etc., with the fixing device capable of fixing an unfixed toner image using electrophotography.
2. Description of the Related Art
A fixing device employing an electromagnetic induction heating system is widely known, which reduces a startup time period needed in an imaging formation apparatus, such as a copier, a printer, etc., to save energy. For example, a fixing device of the Japanese Patent Application No. 2006-350054 (JP-2006-350054-A) employs an electromagnetic induction heating system and mainly consists of a supporting roller (e.g. a heating roller) as a heater, an auxiliary fixing roller (e.g. a fixing roller), a fixing belt stretched by the supporting roller and the auxiliary fixing roller therearound, an induction heating unit (e.g. an induction heating device) opposed to the supporting roller via the fixing belt, and a pressing roller contacting the auxiliary fixing roller via the fixing belt, etc. The induction heating unit mainly consists of a coil unit (e.g., an excitation coil) wound in a longitudinal direction and a core (e.g., an excitation coil core) opposed to the coil unit.
The fixing belt is heated at a position opposite an induction heating unit and heats and fixes a toner image on a recording medium conveyed to a position between the auxiliary fixing roller and the pressing roller. Specifically, by supplying an alternating high-frequency current to a coil unit and thereby forming an alternating magnetic field therearound, an eddy current is generated near a surface of the supporting roller. When the eddy current is generated, the supporting roller generates Joule heat by its own electrical resistance as a heater. Due to the Joule heat, the fixing belt wound around the supporting roller is heated. Since the heater is directly activated by the electromagnetic induction, it is known that the fixing device with an electromagnetic induction heating system like this has a higher thermal effectiveness and is capable of increasing a surface temperature (i.e., a fixing temperature) of a fixing belt to a prescribed level achieving quick startup with less energy than a conventional system with a halogen heater or the like.
A coil unit used in the induction heating system mainly consists of an excitation coil and a core for guiding an alternating magnetic field arising from the excitation coil. FIG. 22 shows a cross-sectional view of a fixing device using a conventional technology described in JP-2006-350054-A. As shown there, from a coil 25 to a long supporting roller 23 that doubles as a roller type heater, multiple arch-type cores 26 are placed in a lengthwise direction thereof covering the coil in a dome shape, thereby forming a continuous magnetic circuit. Further, since a magnetic channel to the heater is insufficient if formed only by the arch-type cores 26, a side core 26b and/or a center core 26a are additionally employed to reduce leakage of an alternating magnetic flux to improve heat generation effectiveness.
Further, in the fixing device described above, a pair of side cores 26b is arranged parallel to each other or parallel to a secondary hold unit 20 that functions as a part of a housing of the fixing device 19. However, the side core 26b is not extended along a radial line drawn from an axis of the supporting roller 23 in a radius direction. In addition, an end face of the side core 26b placed opposite an outer circumferential surface of the supporting roller 23 is not perpendicular to the radial line. Accordingly, leakage of magnetic flux occurs, and accordingly heat generation effectiveness deteriorates due to the presence of a magnetic flux not passing through the supporting roller.
As described later in detail with reference to FIG. 8, it has been found through experiment that the heat generation effectiveness of the induction heating system can be upgraded if the core is placed to increase an area of the core opposite the supporting roller of the heater and reduce the leakage of the magnetic flux not passing through the supporting roller.
Japanese Patent Application Publication No. 2000-056603 (JP-2000-056603-A) discloses a technology in which an opposed surface of a ferromagnetic core opposite a fixing roller as a heater is molded to a prescribed shape almost parallel to a fixing roller to increase an area of the opposed surface thereof. A magnetic flux caused by an excitation coil concentrates within a space between leading ends of protruding portions of the ferromagnetic core, so that leakage of the magnetic field outside of a magnetic circuit, which mainly consists of the ferromagnetic core and a conductive layer on a fixing roller, is decreased. However, forming the opposed surface opposite the magnetic core made of ferrite in parallel to the fixing roller is generally difficult and costly. For molding the ferrite core itself, a method of baking and hardening ferrite powder in a mold is usually employed. However, a problem caused by this manufacturing method is that the core shrinks during a sintering process, and accordingly its dimensional accuracy is degraded.
In addition, highly accurate dimensioning is needed for locating a surface of the core opposite the fixing roller due to the shape of the fixing roller, and as a result, the fixing unit cannot be assembled when the dimensional accuracy is poor. To avoid this problem, the surface of the core opposite the fixing roller must undergo additional processing, such as cutting, etc., thereby increasing manufacturing cost.